In order to express a target gene in a host cell, an expression vector and a gene transfer technique for carrying a structural gene of interest and expressing the same within the cells are required. In this regard, the expression vector capable of expressing a DNA fragment inserted therein generally includes regulatory elements, such as a promoter or an enhancer. Such regulatory elements facilitate the expression of a target gene operably linked thereto. The expression vectors can be selected depending on host cell type, target gene expression level, type of expression desired and the like, and a variety of expression vectors have been developed to satisfy the desired purposes.